Bone cement system and method

ABSTRACT

A bone cement system and related methods useful in flowing liquid bone cement into a prepared long bone medullary canal prior to implanting a prothesis stem in the canal.

The invention relates to the components of a bone cement system fordelivering liquid bone cement into a prepared long bone medullary canaland for sealing the distal end of the canal and to related methods. Thecomponents and methods are useful during implantation of a prothesisstem in the canal. The components include an improved umbrella-typedelivery device, an improved pressure sealing intramedullary plug forclosing the distal end of the canal and a holding and loading device forthe plug.

Murray U.S. Pat. No. 4,466,435 discloses an umbrella-type deliverydevice for flowing liquid acrylic bone cement into a prepared long bonemedullary canal before the stem is implanted in the canal. Thecharcteristics of liquid bone cement, techniques for using the deliverydevice and the advantages of the umbrella-type device are discussed inthe patent.

Conventionally, the distal end of the prepared medullary canal isplugged prior to filling the canal with liquid bone cement. The purposeof the plug is to limit the extent of the canal which is filled withcement. The cement is pressurized as it is flowed into the canal andalso during insertion of the prothesis stem into the filled canal. Theplug must effectively seal the canal to maintain the pressure of theliquid cement and prevent the cement from flowing distally past theplug.

Three kinds of plugs are presently used to seal the distal end of thefemoral canal: an acrylic cement plug, a molded plastic insert plug anda plug fashioned from the live bone.

The acrylic cement plug is a mass of liquid acrylic cement introducedinto the distal end of the prepared canal and maintained in place untilit hardens. The plastic, typically molded from polyethylene, ispress-fitted in place. The bone plug is fashioned from a portion ofcompatible bone, preferably oversized with regard to the canal end, andis then moved through the canal into the desired location.

A number of problems are associated with the conventional plugs. Thecement required for the acrylic plug must be mixed and then introducedwhile liquid into the distal end of the canal. The canal may notsubsequently be filled with liquid bone cement until sufficient time haspassed for the plug cement to harden in place and close the end of thecanal. A bone plug must be sized and then fitted in place. Both these ofprocedures take considerable operating room time. Also, it is difficultto assure that the desired close fit between a bone plug and thesurrounding bone is achieved. Pressure blow-out or dislodgement ofacrylic and bone plugs does occur.

Use of the acrylic plug runs the real risk that withdraw of the cementdelivery nozzle following filling of the canal with liquid cement candraw the cement and plug proximally with the nozzle, thereby breakingthe plug-canal seal and reducing the length of the filled canalavailable for placement of the prothesis stem.

Conventional molded plastic plugs are weakly anchored to the surroundingbone and may be dislodged when the liquid cement in the canal ispressurized during delivery, withdrawal of the nozzle or insertion ofthe prothesis stem.

Plastic and bone plugs cannot be used for closing the end of a medullarycanal located distally of a narrow point having a cross section smallerthan the cross section at the plug site. Additionally one molded plasticplug will not close all size medullary canals commonly experienced inperforming a given procedure, thus requiring that an inventory ofdifferent sized molded plugs be maintained in order to assure that anappropriate size plug is available as required.

The umbrella-type delivery device of the invention is an improvementover the device disclosed in U.S. Pat. No. 4,466,435 and includes ashield with ribs and webbing for improved collapse of the shield beforeinsertion of the device into the canal and subsequent expansion of theshield within the canal for improved pressure engagement with the canalwalls. The pressure of the liquid cement flowed through the device andinto the canal overcomes the frictional engagement between the shieldand the canal so that the device is automatically expelled aspressurized liquid bone cement fills the canal. A specialized keeper isprovided to stress the ribs for spring engagement against the walls ofthe canal.

The invention also includes an improved pressure-type intramedullaryplug for sealing the distal end of the canal. The plug includes anexpandable umbrella-type shield similar to the shield of the improveddelivery device which, when collapsed, may be carried in the lead end ofthe delivery device for movement through the canal to the sealed distalend. The plug is released from the delivery device to permit the plugshield to expand and form a large area contact with the canal wall andthereby close the canal end. The shield of the delivery device is alsoreleased at the same time and likewise expands to engage the walls ofthe canal proximally of the expanded plug.

Liquid bone cement flowed through the delivery device and into the canalbetween the shields pressurizes the interior of the expanded plug shieldand biases the exterior of the plug shield against the wall of thecanal. The axial length of the contact between the plug shield and thecanal wall is sufficient to prevent pressure movement of the plugdistally, despite expected variations in the diameter of the canal. Inthis way, only a single plug will fit canals having a wide range ofdiameters.

The pressure of the liquid cement confined between expanded shields ofthe plug and delivery device retains the plug in place whileautomatically expelling the device proximally along the canal. Theproximal surface of the device shield cleans the wall of the canal topromote the desired intimate and large area contact between thepressurized liquid cement and surrounding bone, as explained more fullyin U.S. Pat. No. 4,466,435.

The medullary canal plug is preferably stored in a plug holder whichconfines the plug shield ribs in an expanded position greater indiameter than the anticipated maximum diameter of the canal at thedistal end. Prior to plugging and filling the canal with cement the endof the delivery device collar is inserted into the holder whichautomatically collapses and inserts the plug into the collar formovement, while collapsed, through the canal to the distal end. Thecollapsed plug is easily moved past narrow points in the canal whichwould obstruct conventional large molded plastic or live bone plugs.

Other objects and features of the invention will become apparent as thedescription proceeds, especially when taken in conjunction with theaccompanying drawings illustrating the invention, of which there are 4sheets.

IN THE DRAWINGS

FIG. 1 is a side view illustrating a bone cement dispenser with anumbrella-type delivery device according to the invention;

FIGS. 2 and 3 are sectional views of the free end of the delivery deviceshowing the shield in different positions;

FIG. 4 is a view taken along line 4--4 of FIG. 3;

FIG. 5 is an enlarged view of the lip of the shield of FIG. 4;

FIG. 6 is a view taken along line 6--6 of FIG. 2;

FIG. 7 is a perspective view of a keeper shown in dotted lines in FIG.3;

FIG. 8 is an exploded view of a holder for a intramedullary plug;

FIGS. 9 and 10 are cross sectional views of the plug holder illustratingnormal retention of the intramedullary plug and insertion of the pluginto the end of the delivery device; and

FIGS. 11 and 12 illustrate use of the plug and dispensing device inflowing liquid bone cement into a medullary canal.

Improved umbrella-type delivery device or interfacer 10 includes anumber of elements like those of the similar delivery device of U.S.Pat. No. 4,466,435. The description of the present device usesterminology which in some respects differs from the terminology used indescribing the prior device. In the present description, old terminologywill be set forth in parentheses for puroses of clarification.

The delivery device (nozzle) 10 includes a liquid cement delivery nozzle(tube) 12, with an expandable tip 14 on the free or distal end thereofand an enlarged cap 16 on the other or proximal end thereof as shown inFIG. 1. Cylindrical collar 18 surrounds the nozzle 12 and is slightlyshorter in length than the nozzle. The collar has a sliding fit on thenozzle so that it may be moved distally and proximally on to the nozzleby gripping expanded hexagonal handle 20. Indicia 22 are spaced alongthe length of the collar adjacent the handle to permit the surgeon tojudge the depth of insertion of the delivery device within the medullarycanal.

The device cap 16 includes means for attachment to a reservoir of liquidbone cement carried by bone cement dispenser 24 which, when actuated,flows pressurized liquid bone cement from the reservoir through the capand nozzle and outwardly of the device through tip 14.

Delivery device tip 14 is preferably molded from a plastic material andincludes a cylindrical tip tube 26 and an expandable bone cement shield28 extending from one end of the tip tube 26. The shield includes aplurality of circumferentially spaced ribs 30 extending from the end ofthe tip tube to the shield lip 32. Flexible plastic sheeting 34 extendsbetween adjacent ribs 30 from the end of the tip tube to the lip so thatwhen the shield is freely expanded the straight ribs 30 projectoutwardly of the tube and the sheeting is taut between the ribs. Theinner ends of the ribs are joined to the tip tube by reduced crosssection hinge connections 36.

As illustrated in FIGS. 4 and 5, the ribs 30 are triangular intransverse cross section and each includes a side 38 on the interior ofthe shield lying approximately in a radial plane with all of the ribssides 38 facing in the same circumferential direction. Each rib 30 alsoincludes an interior side 40 facing to the opposite circumferentialdirection and intersecting the adjacent side 38 at an acute angle. Eachsheeting portion 34 between two ribs is generally triangular in shape asillustrated in FIG. 4 and extends between the surfaces 38 and 40 of theribs 30. The sheeting of portions 34 includes a triangular relativelythick portion 42 extending from adjacent rib surface 38 and a relativelythin sheeting portion 44 extending from the edge of portion 42 to theadjacent rib surface 40. Each of the sheeting portions 40 and 42 extendsalong the ribs from lip 32 to the junction between the shield and thetip tube 26 and are joined mid-way between the ribs at boundary 52.

The tip 14 is secured to the distal end of nozzle 12 by inserting thetip tube 26 within the end of the nozzle and forming a suitablepermanent bond between the tip tube and nozzle.

The fully expanded molded tip 14 may be passed through a suitablecollapsing die which includes surfaces for forcing the ribs andtriangular sections of flexible sheeting 34 inwardly to collapse theshield and fold portions 42 and 44 onto themselves and form U-shapedfolded pleats 48 as shown in FIG. 6 between the collapsed and generallyparallel axially extending ribs 30. When the ribs are collapsed to thisposition the surfaces 38 and 40 of adjacent ribs generally parallel toeach other and define a series of pleat-confining slots 50 between theribs, all of which extend in the same non-radial direction from theouter circumference of the collapsed shield. The boundary 52 betweensheeting portions 42 and 44 is about equal distant between the adjacentribs 30 so that when the sheeting 34 between the ribs is collapsed toform the pleats sheeting portions 42 form one side of the pleatsadjacent surfaces 38, portions 44 form the other side of the pleatsadjacent rib surfaces 40 and the portions are folded over each otherwithin the thinner or more flexible sheeting portions near theboundaries 52. The boundaries are at the crests of the pleats. Theangled slots 50 permit efficient use of the limited available storagespace such that the pleats may have a collapsed length at the shield lipgreater than the interior radius of the collar. Orderly collapse of thesheeting permits a shield having a relatively large diameter whenexpanded to be collapsed into a relatively small diameter configuration.The collapse of each sheeting portion 34 into a like pleat 48 assuresthat each of the sheeting portions opens properly when the shield isexpanded to form a proper pressure seal between the shield and thesurrounding wall of the medullary canal.

As shown in FIG. 2, when the tip 14 is secured to the end of nozzle 12the bone cement shield 28 may be collapsed within the extended free endof the surrounding collar 18. In practice it has been found advantageousto maintain the shield in the position of FIG. 3 during the intervalprior to use for delivery of cement into a medullary canal. In FIG. 3the collar 18 has been withdrawn from the position of FIG. 2 proximallysufficiently to position the free collar end 54 about one-third of thelength of the ribs distally of the hinge connections 36 joining the ribsand the tip tube. The hinge connections are located at or beyond thefree end 56 of nozzle 12. With the collar in this position a keeper 58,shown in FIG. 7, is inserted into the interior of the shield to expandthe shield and bend ribs 30 outwardly about the collar end 54. See FIG.3.

Keeper 58 is generally funnel-shaped and includes a cylindrical pilot60, a conical shaping member 62 and a circumferential lip 64 at thesurrounding the major end of the shaping member.

As illustrated in FIG. 3, when the keeper 58 is inserted into theinterior of the bone cement shield 28 the pilot 60 extends into the tiptube and the conical shaping member 60 engages the portions of the ribsoutwardly of collar end 54 and flexes them outwardly against the end,which forms a fulcrum. The pleating between the outwardly bent portionsof the ribs, is at least partially unfolded. When fully inserted, thekeeper lip 54 meets the ends of the ribs and protects the shield lipprior to use.

Following manufacture of the delivery device 10 a keeper 58 is insertedinto the nozzle as shown in FIG. 3 so that during the storage intervalprior to use ribs 30 are confined in a shallow V-shape as shown in FIG.3. While held in this position the plastic ribs experience cold flow andtend to assume the V-shape in which they are confined. Before thedelivery device is used the keeper is removed and the bone cement shieldis collapsed by distal extension of the collar as in FIG. 2 and the ribends are biased outwardly against the collar so that they will snap openafter the collar is withdrawn. This assures that the shield willproperly open within the canal. Further, the expanded ribs will beforced outwardly against the wall of the canal thereby improving theshield-wall seal. This feature improves the ability of the shield 28 tomake an effective seal with different diameter medullary canals so thata single delivery device may be reliably used to deliver bone cementinto medullary canals of different diameters encountered in performing agiven procedure.

Initial collapse of the bone cement shield 28 is achieved by moving theshield through the collapsing die as described. Subsequent collapse ofthe shield by the collar is achieved by pushing the ribs radiallyinwardly. As the collar is moved distally the end 54 pushes the ribsinwardly and the relatively thin sheeting portions 44 are first foldedinwardly towards the adjacent rib surface 40 while the relatively thicksheeting portions 42 resist being bent with respect to its adjacent rib30. Further rib collapse then folds the thick sheeting portions 42inwardly with respect to the adjacent ribs 30, thereby completing thepleats as shown in FIG. 5 and assuring that each pleat is U-shaped asdescribed with the webbing portions 42 and 44 forming the pleat sides.

FIG. 6 illustates that the relatively thick sheeting portions 42 resistbeing bent relative to their adjacent rib surfaces 40 and hold therelatively thin sheeting portions 44 closely against surfaces 40 oftheir adjacent ribs 40.

The bone cement shield of discharge device 10 is provided with eightribs 30. It is contemplated that the shield may be provided with agreater number of equally spaced ribs. Such a shield would have theadvantage of reducing the circumferential length of each portion offlexible sheeting between ribs. By reducing the length of the portionsat the circumference of the shield the maximum length of the pleats isreduced, making storge of the pleats easier within the interior of thesurrounding collar. For instance, increasing the number of ribs from 8to 12 would markedly reduce the maximum circumferential length of eachsheeting portion making it easier to fold and store the pleats withinthe limited interior of the collar.

The tip 14 may be molded from a polyurethane plastic possessingsufficient strength and elastomeric properties to permit the shield tobe collapsed and expanded and to form the desired pressure seal with themedullary canal as described.

The delivery device 10 disclosed herein is particularly useful inflowing liquid bone cement into the interior of prepared femoralmedullary canals prior to mounting a femoral prothesis within the canal.The normal diameter expected in an adult femoral medullary canal has arange of from 10 to 18 millimeters or about 0.39 to 0.71 inch. A singledelivery device 10 may be used for reliably flowing cement into canalswithin this range.

In the device 10 the outside diameter of the tip tube is about 0.33inch. The straight length of rib 30 is about 0.5 inch. When the shieldis fully expanded the ribs diverge from the shield axis at 30 degreesand the maximum diameter of the shield at lip 32 is 1.33 inch. Therelatively thin sheeting portion 44 has a thickness of about 0.006 inchand the relatively thick sheeting portion 44 has a thickness of about0.015 inch, 21/2 times the thickness of the thin sheeting. Rib surfaces38 and 40 intersect at an angle of about 45 degrees. The fully collapsedshield shown in FIG. 2 is confined within the interior of collar 18 hasan interior diameter of about 0.38 inch. The outside diameter of thedistal end of the delivery device 10 is about 0.44 inch.

The keeper conical shaping member 62 has sides which diverge from itsaxis at an angle of 45 degrees so that when the keeper is inserted intothe partially confined shield as shown at FIG. 3 the free portions ofthe ribs 30 are bent away from the shield axis 45 degrees in order tobend the ribs into the shallow V-shape previously described.

FIG. 8 includes a prospective view of an intramedullary canal plug 80useful in pressure-sealing the distal end of a medullary canal tomaintain the pressure of liquid acrylic bone cement within the canal andprevent the cement from flowing distally past the plug. Plug 80 includesa bone cement shield 82 and a solid base 84 at one end of the shield.The shield 82 is similar to the delivery device shield 28 previouslydescribed and includes 12 elongate ribs extending from the base to theshield lip 86, hinge connections between the ribs and the base anddifferential thickness webbing portions between the ribs as in shield28. The diameter of the base is slightly smaller than the interiordiameter of the collar 18. The ribs are 0.75 inch long, 11/2 times thelength of ribs 30. When the plug is fully expanded the maximum diameterat the shield at lip 86 is 1.83 inch. When fully expanded, the ribsextend outwardly of the plug axis at an angle of 30 degrees. The fullyexpanded plug has a maximum diameter at lip 86 of 1.83 inch. The ribs ofshield 82 are triangularly shaped like ribs 30 and cooperate with thedifferential thickness flexible sheeting between the ribs for controlledcollapse of the shield with the formation of U-shaped pleats betweenadjacent ribs when collapsed. When fully expanded, shield 82 assumes aconical shape with the straight ribs extending 30 degrees from the plugaxis. The ribs of shield 82 are longer than the ribs of shield 28 inorder to assure that the plug is immovably pressure sealed to themedullary canal wall during flowing of bone cement into the canal.

The intramedullary plug base 84 is cylindrical in transverse crosssection, but unlike the tip tube is solid, without a central opening. Inthis way, the expanded plug effectively closes off the distal end of themedullary canal.

Plug 80 may also be molded from a strong, flexible plastic material suchas polyurethane. The plug is molded in an expanded, generally conicalshape and then moved through a collapsing die to fold the sheetinginwardly between the ribs in the same manner as described in connectionwith the shield 28. The plug is then positioned within the plug holder88 shown in FIGS. 8 and 9.

Plug holder 88 includes base 90, an elongate cover 92, collapser element94 and spring 96. Plug holder base 90 includes a cylindrical post 98, acircular flange 100 at one end of the post with a step 102 at thejunction between the flange and the cylindrical post. A shallow bore 104is provided on the other end of post 98 for receiving and holding plugbase 84. Bevel surface 106 extends from the side of post 98 inwardly tothe end of bore 104.

Cylindrical cover 92 includes a cylindrical interior recess 108' openingat one end such that the cover may be fitted over the base with the openend fitted snugly around step 102 to assure the cover is maintainedcoaxial with the base. A beveled lead-in opening 110 is provided in theother end of the cover away from the base and has a minimum diameterless than the diameter of recess 108'.

Collapser member 94 has a close sliding fit within bore 108' and isbiased toward the opening 110 by spring 96. The spring is fitted overpost 98 and is confined between the step 102 and the outercircumferential end 112 of the collapser member. The collapser memberincludes a first circular recess 114 having a diameter approximatelyequal to the interior diameter of the spring 96, a minimum diameterrecess 116 spaced from recess 114 and having a diameter slightly lessthan the interior diameter of the delivery device collar 18, a beveledshield collapse recess 118 joining recesses 114 and 116 for collapsingthe shield 82 and an end recess 120 located between recess 116 and theend of the collapser member 94 away from base 90. The diameter of recess120 is slightly greater than the exterior diameter of collar 18. Theouter end of recess 120 is provided with a lead-in champer 122 whichforms a continuation of bevel surface 110 when the collapser member isheld against the end of the cover as shown in FIG. 9.

The intramedullary plug 80 is confined within the plug holder 88 asshown in FIG. 9 with the plug base 84 fitted within bore 104 and thebone cement shield confined in a partially expanded position by thecoils of spring 96 and bore 114. The ribs are molded straight so thatwhen the plug is collapsed and confined as shown the ribs are stressedand resiliently hold the shield against the spring and bore therebyassuring that after the plug is collapsed and seated within the deliverydevice collar and released in the distal end of the canal the shieldwill snap open and the ribs will hold the shield against the canal wallfor forming an immovable pressure seal. During storage of the plugwithin the holder the ribs tend to cold flow and assume the confinedposition thereby assuring that the released plug will snap open. In thecase of a intramedullary plug used to close the proximal end of afemoral canal, the interior diameter of the spring and of recess 114 maybe about 0.75 inch which is greater than the maximum expected diameterof the canal.

The expanded intramedullary plug 80 confined in holder 88 is collapsedand loaded into the lead or distal end of delivery device collar 18immediately after the device bone cement shield 28 has been collapsedand loaded into the collar as shown in FIG. 2. The collar is extendeddistally from the position of FIG. 2 so that end 54 is beyond thecollapsed device bone cement shield 28 a distance sufficient to receivethe collapsed plug 80. The distal end of the collar is then positionedas shown in FIG. 9 and pushed into the plug holder through lead-ins 110and 122 until it seats on step 124 at the end of end recess 120. Thediameter of adjacent recess 116 is slightly less than the interiordiameter of collar 18. With the base 90 supported against movement thecollar is then forced toward the base as indicated in FIG. 10 so thatthe collapser member 94 is moved to the right and spring 96 iscompressed. With plug base 84 bottomed in bore 104 the plug is heldagainst movement so that the lip 86 of the partially collapsed plugshield 82 is brought into contact with the shield collapse recesssurface 118 and moved radially inwardly and ultimately extended into theminimum diameter recess 116. During collapse of the plug shield therelatively thick and relatively thin sheeting portions between adjacentribs are folded together as described in connection with the collapse ofthe device shield 28 to form U-shaped pleats which are confined withinangled recesses between adjacent ribs upon full collapse.

As the collapser member 94 moves to the right the collapsed shield isloaded within the interior of the collar adjacent the previously loadeddevice shield 28 as indicated in FIG. 10. Loading is completed when thecollapser member 94 bottoms on the end of post 98. The loaded device 10may then be freely withdrawn from the plug holder 88. The base and partof the collapsed plug shield may extend beyond collar end 54.

After loading of plug 80 into the distal end of the extended collar 18the surgeon moves the distal end of device 10 into the medullary canal,using indicia 22 to determine when the device is inserted to a properdepth. The surgeon then grips handle 20 and fully pulls back the collarwhile holding the dispenser 24 fixed so that collar 18 is withdrawn tothe position of FIG. 12. Withdrawal of the collar substantiallysimulataneously releases both of the plug and device bone cement shields82 and 28 for expansion in the distal end of the canal as illustrated inFIG. 12.

The ribs of the released bone cement shields are biased outwardlyagainst the interior wall of the canal as illustrated. Liquid acrylicbone cement 128 is then flowed from dispenser 28 through the interior ofnozzle 12 and outwardly through the tip 14 into the space 132 betweenthe expanded bone cement shields. The pressurized cement biases theexpanded shields against the canal wall 126 and may capture an airpocket 130 at the distal or base end of the plug. During initial flowingof the pressurized cement into the space 132 a limited amount of cement134 may flow past the shields at small apertures between the ribs. Theseapertures are immediately plugged by the cement so that the pressurebuilds up in space 132 and forces the ribs and sheeting out against thewall 126, thereby maintaining a full pressure differential betwen theshields and the wall.

The bone cement shield 82 of plug 80 contacts the circumference of wall126 for an axial distance indicated by 136 in FIG. 12. The bone cementshield 28 of delivery device 10 has circumferential contact with wall126 for an axial distance indicated by 138, less than distance 136.

The pressure of the cement 128 within space 132 biases the shieldsagainst the wall 126 along a circumferential area determined by thediameter of the canal and the axial distance the shield contacts thecanal.

Pressurization of the interior of a bone cement shield by liquid bonecement will exert an axial force on the shield tending to move theshield along the canal and will also force the shield against the wallof the canal and generate a frictional force resisting movement alongthe canal.

The axial force applied to the shield is:

    F.sub.axial =(P.sub.c) (pi) (D.sub.c /2).sup.2 ;

where P_(c) is the pressure of the bone cement, D_(c) is the diameter ofthe canal at the shield and pi as used in this and subsequent formulas,is the ratio of the circumference of a circle to its diameter.

The maximum frictional force resisting movement of the shield is:

    F.sub.friction =(L.sub.a) (pi) (D.sub.c) (P.sub.c) (mu);

where L_(a) is the axial length of contact between the shield and thecanal wall, D_(c) is the diameter of the canal at the shield, P_(c) isthe pressure of the cement in the shield and mu is the co-efficientfriction between the shield and the wall.

The pressurized cement will not move the shield along the canal when theaxial force F_(axial) is less than or equal to the maximum frictionforce resisting movement, F_(friction), or:

    (P.sub.c) (pi) (D.sub.c /2).sup.2 ≦(L.sub.a) (pi) (D.sub.c) (P.sub.c) (mu); or

    D.sub.c /L.sub.a ≦4 mu

Conversely, in order to assure that a bone cement shield is moved alongthe canal by the pressure of the liquid cement flowed into the canal theaxial pressure exerted on the shield, F_(axial), must be greater thanthe maximum frictional force resisting movement between the shield andthe canal wall or:

    D.sub.c /L.sub.a >4 mu

It will thus be seen that for a given diameter canal a shield on the tipof a delivery device must have a short L_(axial) in order for thepressurized cement to expel the device from the canal while aintramedullary plug must have a relatively long L_(axial) to assureagainst pressure dislodgement. Expulsion or pressure retention of thebone cement shield is not dependent on the pressure of the cement.

The long ribs and sheeting between the ribs of intramedullary plug 80assure that the end of the plug bone cement shield 82 has an annulararea of pressure contact 136 with the canal wall sufficiently long toassure enough friction that the axial force exerted on the plug by thepressurized bone cement is incapable of dislodging the plug distally,independent of the pressure of the liquid bone cement. The pressurizedliquid cement maintains the plug in place to seal the distal end of thecanal.

The axial length of the engagement between the bone cement shield 28 ofdevice 10 is considerably shorter than that of the intramedullary plug80 so that, applying the formulas above, the axial force exerted on theshield 28 and the bone cement reservoir exceeds the frictional forcerestraining movement of the field in the canal so that the pressurizedcement will force the shield, device and dispenser 24 axially proximallyof the canal as pressurized liquid bone cement is flowed into the space132 between the shields.

During the filling of the canal the surgeon actuates the dispenser 24 tomaintain the pressure of the bone cement. Higher pressure may beachieved by actuating the device and manually resisting expulsion ofdevice 10 from the canal. Movement of the bone cement shield 28 alongthe canal surface 126 as the canal is filled with pressurized bonecement cleans and scrapes the canal surface so that the liquid cement onthe high pressure side of the shield flows into the interstatices of theclean surface, as explained more fully in U.S. Pat. No. 4,466,435. Afterthe canal is filled with cement and device 10 has been expelled aprothesis stem is implanted in the canal in a conventional manner.

An intramedullary plug 80 may be positioned in the end of a conventionalbone cement nozzle without a bone cement shield using a plug holder 88.The empty end of the conventional nozzle is pushed into the holder tocollapse the plug and seat it within the nozzle tip. After the nozzle isextended into the canal to position the plug at the distal end, cementis flowed through the nozzle to force the plug out into the canal, whereit expands and seals the end as described.

Alternatively, an expanded plug 80 may be pushed through the canal tothe distal end using an elongate insertion rod, the lead end of which isseated in recess 140 formed in the pressure side of the shield base 84.See FIG. 12. Presurized bone cement subsequently flowed into the canalassures that the plug immovably seals the distal end of the canal.

An additional advantage of this plug is that at time of prothesisinsertion into the canal the plug can be pushed further distally by theprothesis tip if it was inadvertantly placed at too shallow a depth inthe femoral canal. In contrast, other commonly used plugs cannot bemoved more distally without the strong possibility of dislodging them sothat they no longer are anchored in the canal or without having to exertexcessive and dangerous pressure to move them, or both of theseproblems.

While I have illustrated and described a preferrred embodiment of myinvention, it is understood that this is capable of modification, and Itherefore do not wish to be limited to the precise details set forth,but desire to avail myself of such changes and alterations as fallwithin the purview of the following claims.

What I claim my invention is:
 1. An intramedullary canal plug includinga base imperforate to bone cement and an umbrella-type cement shieldextending from the base, the shield comprising a plurality of ribsjoined at one end to the base, circumferentially spaced around the baseand extending outwardly of the base to a shield lip and a plurality offlexible sheeting portions each extending between adjacent ribs from thebase to the shield lip, each sheeting portion including a firstrelatively more flexible part near one adjacent rib and a secondrelatively less flexible part near the other adjacent rib whereby uponcollapse of the shield the first sheeting parts fold inwardly before thesecond sheeting parts fold inwardly.
 2. A plug as in claim 1 wherein allsaid first parts are on the same circumferential side of the sheetingportions so that such portions collapse in a like manner.
 3. A plug asin claim 2 wherein the thickness of the first sheeting part is less thanthe thickness of the second flexible sheeting part.
 4. A plug as inclaim 3 wherein each flexible sheeting portion is generally triangularin shape with long sides joining the adjacent ribs and a short sideforming a portion of the shield lip when the shield is expanded and theparts are generally triangular in shape, join each other at a boundaryapproximately midway between the adjacent ribs and extend from thedistal end of the nozzle to the shield lip.
 5. A plug as in claim 4wherein each rib includes a first side on the interior of the shieldlying approximately in a radial plane and facing in a firstcircumferential direction and a second side on the interior of theshield extending toward the first side at an acute angle and facing inthe opposite circumferential direction whereby upon collapse of theshield with the ribs extending in a parallel direction from the base thesides of adjacent ribs parallel each other and define like non-radialsheeting confining slots, said sheeting being folded in pleats in saidslots.
 6. A plug as in claim 5 wherein said ribs are generallytriangular in cross section and the sides extend toward each other atangles of about 45 degrees.
 7. An intramedullary canal plug including abase imperforate to bone cement and an umbrella-type cement shieldextending from the base, the shield comprising a plurality of ribsjoined at one end to the base, circumferentially spaced around the base,and extending outwardly of the base to a shield lip and flexiblesheeting portions extending between adjacent ribs, each rib including afirst side on the interior of the shield lying approximately in a radialplane and facing in a first circumferential direction and a second sideon the interior of the shield extending toward the first side at anacute angle and facing in the opposite circumferential direction wherebyupon collapse of the shield with the ribs parallel and extending fromthe base the sides of adjacent ribs parallel each other and define likenon-radial sheeting confining slots, said sheeting being folded inpleats within said slots.
 8. A plug as in claim 8 wherein the interiorsides of said ribs extend toward each other at angles of about 45degrees.
 9. A device for plugging and flowing bone cement into a longbone medullary canal including a tubular nozzle, an umbrella-type bonecement shield at the distal end of the nozzle, means at the proximal endof the nozzle for attaching the device to a source of bone cement sothat cement may be flowed from the source through the device and intothe canal, a collar surrounding the nozzle and having a distal endlocated a distance beyond the distal end of the nozzle shield to definean interior space at the end of the collar, the nozzle shield beingcollapsed and confined within the collar proximally of said space, andan intramedullary canal plug including a base and an umbrella-type bonecement shield extending away from the base, said plug being fittedwithin the space at the distal end of the collar with the collapsednozzle and plug shields adjacent each other whereby upon positioning thenozzle within a prepared medullary canal and moving the collarproximally relative to the nozzle both shields are releasedsubstantially simultaneously within the canal.
 10. The device of claim 9wherein the plug base is located outwardly of the distal end of thecollar.
 11. A holder for an intramedullary canal plug of the type havinga base, an expandable umbrella-type cement shield and a shield lip, theholder comprising a holder base with first means for holding the plugbase; a collapser member normally spaced from the first means andincluding (a) a relatively large shield-holding recess facing the firstmeans, (b) a shield collapsing recess having a large area end adjacentthe shield-holding recess and a reduced area end remote from theshield-holding recess, (c) an end recess larger than the reduced areaend of the shield collapsing recess, and d) a step joining the endrecess and the reduced area end of the shield-collapsing recess; andsecond means supporting the collapser member spaced from the holder baseand permitting movement of the collapser member toward the holder basewhereby an expanded intramedullary canal plug positioned within theholder with the plug base in said first means and the lip of theexpanded shield in the shield-holding recess may be collapsed and loadedin the end of a collar by seating the collar within the end recess andmoving the collar toward the holder base to force the collapser membertoward the holder base as the shield is collapsed by the shieldcollapsing recess and loaded into the end of the collar.
 12. A holder asin claim 11 wherein said second means includes a sliding connectionjoining the base and collapser member; and a spring biasing the base andcollapser member apart.
 13. A holder as in claim 12 wherein saidrecesses are symmetrical about a central axis.
 14. The method of fillingthe medullary canal of a live bone with bone cement comprising the stepsof:a. Freely introducing an intramedullary canal bone plug with acollapsed bone cement shield into the distal end of the canal; b. Freelyintroducing a bone cement delivery device with a collapsed bone cementshield into the canal with the delivery device shield position adjacentthe plug shield; c. Substantially simultaneously expanding both shieldswithin the canal so that the plug shield contacts the canal wall at thedistal end of the canal and closes the canal and the delivery deviceshield contacts the canal wall proximally of the plug shield to define aspace within the canal between the expanded shields; d. Flowingpressurized bone cement distally through the bone cement delivery deviceshield and into the space between the shields so that the cement biasesthe shields into pressure contact with the canal wall; e. Maintainingthe plug in place at the distal end of the canal during flowing ofpressurized liquid cement into the space; f. Moving the delivery deviceshield proximally along the canal as cement is flowed into the spacewhile biasing the shield against the canal wall to prepare the wall toreceive bone cement; and g. Flowing pressurized bone cement in the spaceinto the interstices of the prepared wall.
 15. The method of claim 14wherein the introducing steps comprise simultaneously moving theintramedullary bone canal bone plug with collapsed shield and the bonecement delivery device with collapsed shield into the canal.
 16. Themethod of claim 15 including the steps of:h. Maintaining the followingrelation at the plug shield:

    D.sub.c /L.sub.a ≦4 (mu),

where D_(c) is the diameter of the canal at the plug shield, L_(a) isthe axial length of the circumferential pressure contact between theplug shield and the wall of the canal and (mu) is the coefficient offriction between the shield and the wall of the medullary canal; and i.Maintaining the following relation at the nozzle shield:

    D.sub.c /L.sub.a >4 (mu),

where D_(c) is the diameter of the canal at the nozzle shield, L_(a) isthe axial length of the circumferential pressure contact between thenozzle shield and the wall of the canal and (mu) is the coefficient offriction between the nozzle shield and the canal wall.
 17. The method ofclaim 15 including the step of maintaining the axial length ofcircumferential pressure contact between the plug shield and the wall ofthe canal greater than the axial length of the circumferential pressurecontact between the nozzle shield and the wall of the canal.
 18. Anintramedullary canal plug having an imperforate central base, and anumbrella-type shield, the shield having a lip away from the base, anumber of ribs extending from the base to the lip and flexible sheetingextending between adjacent ribs, the portion of the shield adjacent thelip having a generally cylindrical shape with a diameter Dc and an axiallength of La, and

    Dc/La≦4(mu)

where (mu) is the coefficient friction between the shield and theinterior wall of a prepared medullary canal so that upon positioning ofthe shield within such a canal and placing a body of pressurized liquidbone cement in the shield the cement biases the cylindrical portion ofthe shield against the canal wall and the plug is stable in the canal.